It's this bug: https://svn.boost.org/trac/boost/ticket/12408 Fixed in boost-1.62. HTH, John. On 19/01/2017 17:39, Craig Henderson wrote:
Hello, I am new to Boost.Multiprecision and have come across a result that I cannot understand.
I have the following code to calculate the sum of three cubes, using bigint_t = boost::multiprecision::mpz_int; bigint_t a = 2220422932; bigint_t b = 2218888517; b = -b; bigint_t c = 283059965; c = -c; std::cout << '(' << a << ")^3+(" << b << ")^3+(" << c << ")^3 = "; bigint_t a1 = a * a * a; bigint_t b1 = b * b * b; bigint_t c1 = c * c * c; std::cout << '(' << a1 << ")+(" << b1 << ")+(" << c1 << ") = "; std::cout << bigint_t(a1+b1+c1) << '\n';
and it produces the correct result (2220422932)^3+(-2218888517)^3+(-283059965)^3 = (10947302325566084787191541568)+(-10924622727902378924946084413)+(-22679597663705862245457125) = 30
However, if I use this implementation, with a,b and c assigned the result of the multiplication, a = a * a * a; b = b * b * b; c = c * c * c; std::cout << '(' << a << ")+(" << b << ")+(" << c << ") = "; std::cout << bigint_t(a+b+c) << '\n'; ... I get a very strange result: (24307641127223864542936438774018437376)+(24240519923499804093545671548118385521)+(6419686120902663137493915211500625) = 48554580736844571299619604237348323522
What is going on? Is this an invalid use of the multiplication that I have missed? I get the same result with cpp_int and gmp; using bigint_t = boost::multiprecision::cpp_int; using bigint_t = boost::multiprecision::mpz_int;
I'm using g++ (Ubuntu 5.4.0-6ubuntu1~16.04.4) 5.4.0 20160609, Boost 1.58
Many thanks Craig
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